A lithium secondary battery, which is a type of an electrochemical device, is characterized by a high energy density and thus has been widely used as a power source for portable equipment such as a portable telephone and a notebook personal computer. The capacity of the lithium secondary battery is likely to increase further as the performance of the portable equipment becomes higher. For this reason, it is important to ensure the safety of the lithium secondary battery.
In the current lithium secondary battery, e.g., a polyolefin microporous film (microporous film) with a thickness of about 20 to 30 μm is used as a separator that is interposed between a positive electrode and a negative electrode. The separator may require a so-called shutdown function to improve the safety of the battery. The shutdown function increases the internal resistance of the battery by melting the resin of the separator at a temperature not more than the abnormal heat generation temperature (thermal runaway temperature) of the battery so as to close the pores of the separator. Therefore, the material of the separator is preferably polyethylene having a low melting point.
To improve the porosity and the strength, the above separator may be formed of a uniaxially- or biaxially-oriented film. Since the separator is provided as an independent film, certain strength is needed in view of workability, and the drawing ensures the strength of the separator. In such a uniaxially- or biaxially-oriented film, however, the degree of crystallinity is increased, and the temperature at which a shutdown is to be effected (i.e., the shutdown temperature) is also raised close to the thermal runaway temperature of the battery. Thus, it is hard to say that the margin for safety of the battery is sufficient.
Moreover, the film has been distorted by drawing and may shrink due to residual stress when it is subjected to high temperatures. The shrinkage temperature is very close to the melting point of the resin of the film, namely the shutdown temperature. Therefore, when the polyolefin microporous film is used as a separator, a rise in temperature of the battery has to be prevented by reducing the current as soon as the temperature of the battery reaches the shutdown temperature due to charging anomaly or the like. If the pores are not sufficiently closed and the current cannot be immediately reduced, the temperature of the battery is easily raised to the shrinkage temperature of the separator, so that an internal short circuit can occur.
As a technology for improving the reliability of the battery by preventing a short circuit due to the thermal shrinkage of the separator, e.g., an electrochemical device including a separator that includes a porous base having good heat resistance, inorganic fine particles, and a resin component for ensuring the shutdown function has been proposed (Patent Documents 1 and 2).
Patent Document 1: WO 2006/062153
Patent Document 2: WO 2007/066768
The technologies disclosed in Patent Documents 1 and 2 can provide an electrochemical device that is not likely to cause thermal runaway even in the case of abnormal heat generation and achieves excellent safety.
Patent Documents 1 and 2 also disclose a method for producing a separator by using the inorganic fine particles and applying a slurry in which the inorganic fine particles are dispersed to the base or the like.
However, the inorganic fine particles have a higher specific gravity compared to the media such as water and an organic solvent, and therefore are likely to be settled in the slurry. In particular, if the particle size is 1 μm or less, the fine particles are likely to be agglomerated together. Thus, it may be difficult to maintain a stable dispersion state of the fine particles in the slurry.
When the dispersion state of the fine particles in the slurry cannot be stably maintained, the fine particles are agglomerated or settled during storage of the slurry. When the slurry in which the fine particles are agglomerated or settled is applied to the base or the like, the application tends to be not uniform. Moreover, when the dispersion state of the fine particles in the slurry is particularly unstable, the fine particles are agglomerated or settled in the period of time between the application and drying of the slurry. This may result in non-uniformity of the surface to which the slurry is applied.
The non-uniform application of the slurry reduces the evenness of the separator produced. Consequently, in the lithium secondary battery, the ionic conduction is not uniform within the separator, which may cause a deficiency such as a deposition of lithium during charging of the battery particularly at a high current density. Moreover, if the deposited lithium becomes a dendrite crystal, a short circuit may occur due to the lithium dendrite.
Accordingly, in the slurry used to produce a separator, it is preferable that the stability of the dispersion state of the fine particles or the like is improved to further stabilize the quality of the separator to be produced. In this regard, the technologies of Patent Documents 1 and 2 still have room for improvement.